The interest in solar cell technology has increased over the last years as energy prices steadily reach higher levels and as the drawbacks of using fossil fuel becomes more and more obvious. Moreover, technology breakthroughs imply that large scale production of high efficiency photovoltaic (solar) cells is possible.
For example, rectifying junctions, such as p-n junctions and Schottky diodes, are fabricated in semiconducting materials to make photovoltaic cells that are used in solar cell devices as well as in photodetectors. A photovoltaic cell converts light into electricity as light illuminates the p-n junction and creates pairs of oppositely charged particles: electrons and holes. These charges are separated by the rectifying junction to produce an electrical current. Photodetectors work on a similar principle.
Conventional photovoltaic cells are often planar devices constructed of at least two asymmetrically doped semiconductor layers with front and back contacts. For a conventional photovoltaic cell, light enters between a grid formed by front contacts where it is absorbed by the n-type layer and p-type layer creating electron-hole pairs. The electron-hole pairs are separated by a p-n junction and a voltage develops across the photovoltaic cell. Useful power is obtained by putting a load across the cell contacts and hence photovoltaic cells convert radiation directly into useful electric energy.
One factor that limits the conversion efficiency from light to electricity is reverse current leakage across the p-n junction. For a planar cell, reverse current leakage increases as the area of the p-n junction increases.
There are many ways to reduce undesirable reverse current leakage for a planar cell but whatever methods are used, reduction of reverse current leakage is limited by the area of the p-n junction, and for a planar cell reducing the area simply means the cell is smaller and thus collects less light. By using a different cell geometry (nonplanar) it is possible to reduce the area of the p-n junction without reducing the total area of the cell.
One successful non-planar geometry is the silicon dot-junction photovoltaic cell which is described in e.g. U.S. Pat. No. 4,234,352. In this patent a dot-junction cell is disclosed, where one surface of a silicon substrate has a series of locally doped n+ regions and p+ regions (dot junctions), such that the area of the doped regions is much less than the total area of the dot-junction cell. The p+ regions form the p-n junction with a bulk, and the n+ regions form areas of low resistivity for n-type contacts.
However, even if reverse current leakage is limited, the efficiency of the dot-junction cell is still not optimal. For example, since each dot-junction is a single band gap solar cell, efficiency is limited due to the inability to efficiently convert the broad range of energy that photons possess in the solar spectrum. In the ideal limit, only photons with energy equal to the band gap energy are efficiently converted to electricity. Photons below the band gap of the cell material are lost; they either pass through the cell or are converted to only heat within the material. The energy in the photons in excess of the band gap energy is also lost to heat due to carrier relaxation to the band edges.
Accordingly, even if a dot-junction photovoltaic cell is advantageous with respect to a reduced reverse current leakage, a dot junction solar cell still has a disadvantage in terms of photon energy efficiency, i.e. a large portion of the solar spectrum is lost to heat and not converted into electricity. Further background art is reflected by patent document EP1944811.